Band Compatibility
You can match this band with any Apple Watch Series 4 case of the same size. It also works with all previous versions of Apple Watch, including Apple Watch Series 3. The 40mm band works with the 38mm case; the 44mm band works with the 42mm case.

The pin - tuck sports bands are very secure. I have used the original apple watch since it came out and have only used the sport band that came with it. I have hiked, biked, lifted weights, played with my children, rock climbed, danced....many activities...with out the watch ever coming loose or adjusting. It was very comfortable and the fit was great.

When you purchase an Apple Watch, you get three pieces in terms of the band. The first piece is the part of the (sports) band that has the peg that the other strap attaches to. The other two pieces will be one that is small/medium sized (about 4-5in), and one piece that is medium/large (5-6in). These measurements are estimations but you get two sizes so you can pick the one that fits best for your wrist size. You get all you need to enjoy your Apple Watch right out of the box. Any other customizations require purchasing more bands/accessories etc. Hope this helps!